Composition effect on melting behaviors of Cu-Au-Pt trimetallic nanoalloys

Highlights

• The composition effect on the melting behaviors of nanoalloys was investigated.

• Melting temperature is associated with the atomic distribution of the nanoalloys.

• Au atoms are generally more movable than Cu and Pt atoms in Cu-Au-Pt nanoalloys.

Abstract

In this study, molecular dynamics simulations were performed on 55-atom icosahedral Cu₁₃Au_nPt_42-n, Au₁₃Cu_nPt_42-n and Pt₁₃Au_nCu_42-n (n = 1–41) nanoalloys. In order to present the composition effect on the melting behaviors in detail, three different icosahedral compositions corresponding to Cu₁₃Au₁₃Pt₂₉, Au₁₃Cu₂₉Pt₁₃ and Pt₁₃Au₂₉Cu₁₃ were selected from Cu₁₃Au_nPt_42-n, Au₁₃Cu_nPt_42-n and Pt₁₃Au_nCu_42-n nanoalloy systems. The temperature dependence of the caloric curve, Lindemann index, heat capacity and root mean square displacement methods was analyzed thoroughly to clarify the simulation results. The simulation results showed that the melting temperature is closely associated with the composition and also atomic distribution of the nanoalloys. It was observed that Au atoms are generally more movable than Cu and Pt atoms in Cu-Au-Pt trimetallic nanoalloys with increasing temperature. Moreover, the Lindemann index variations of second and third layers showed that pre-melting of shell occurs especially for Au₁₃Cu₂₉Pt₁₃ and Pt₁₃Au₂₉Cu₁₃ nanoalloys in these three compositions.

Introduction

Interest in nanoalloys arises from their tunable chemical and physical properties depending on size and composition [1], [2]. This specificity of nanoalloys, indicating that the behaviors differ greatly from those of bulk alloys and individual atoms, brings about their usage in a large variety of applications ranging from catalysis to optoelectronics, magnetism, optics and biomedicine [3], [4], [5]. Size and alloying effects provide an important advantage particularly in the use of nanoparticles as catalysts because it is well known that nanoalloys have high surface –volume ratio which enhances the catalytic activity [6], [7], [8]. In this context, it can be used in a small amount of metals which are rare reserve and expensive [9].

Proton exchange membrane (PEM) fuell cells which can adequately convert the chemical energy into electricity through an electrochemical reactions, are promising techonolgy to decrease the subjection on the fossil fuel energy [10]. They are also used as distributed generation sources due to their clean, high energy efficiency and high power density advantages [11]. For commercialization of PEM fuell cells, it needs to overcome some important issues such as high stability, activity, cost reduction and durability improvement [12]. Pt-based nanoparticles and nanoalloys receive a significant amount of interest due to their promising catalytic activities especially for oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) in PEM fuel cells [13]. A clear synergistic effect between Pt and Au atoms was mentioned in some studies show the enhanced durability of Pt-based electrocatalysts with addition of Au transition metal [14], [15]. Compared to bimetallic and monometallic nanoparticles, recently, Pt-based trimetallic nanoalloys have attracted more attention due to their excellent catalytic activities of PEM electrocatalysts [16]. In order to reduce the use of expensive Pt and Au atoms, Pt-based trimetallic nanoalloy catalysts have been developed with addition of a third cheap metal [17]. Thus, Pt-Au-Cu trimetallic nanoalloys are of interest in this study as one of the most promising trimetallic nanoalloy catalysts because of that lattice constants and melting points differences of Cu, Au and Pt atoms are moderately [18].

The phase transition which plays a crucial role to understand thermal behavior of nanoalloy structures has been centeral to theoretical, experimental and also simulation studies by reason of the fact that having knowledge about the temperature at which the structural stability changes is very important issue especially for catalytic applications [19]. One important way to analyze structural changes is to examine the melting process in detail. As is very well known, the melting behaviors of nanoalloys is more complex than the bulk alloys [6]. This is because, the melting of nanoalloys occurs in a range of temperature but bulk matters have a specific melting temperature [20]. This discrepancy of melting behaviors is attributed to size effect. However, there is not sufficient data about the geometric structure and composition effects on melting process of nanoalloys. Accordingly, how the geometric structure and constituent atoms of the nanoalloys effect the melting process is needed to be investigated further.

Core-shell nanoalloys have been discussed in several simulation studies due to the fact that core and shell can change the significant chemical and physical properties which are important in identifing particularly catalytic activities [21], [22]. Icosahedral core-shell nanoalloys with high symmetry have gained increasing interest in consequence of exhibiting a widescale behavior due to their adjustable constituent atoms on the core and shell. Also, constituent atoms of the nanoalloys can change the melting process due their different bulk melting temperatures. By taking all properties above mentioned into consideration, we have focused on melting of icosahedral Cu-Au-Pt nanoalloys with 55 atoms. The choice reason of this size is that 55 is a geometric magic number for icosahedral structures have quasi-spherical shape and close-packed surface at small sizes [23], [24], [25]. Besides, nanoalloys with less than 100 atoms exhibit more complex melting behaviors with dependence of size and composition [7]. Although in many studies, melting process of momometallic nanoparticles and bimetallic nanoalloys with 55 atoms were investigated in detail [7], [8], [20], [26], [27], [28], [29], [30], [31], [32], [33], there is still lacking a study of geometric structure and composition effects on melting process of trimetallic core-shell nanoalloys with 55 atoms.

Thus, in this study the composition effect on the melting behaviors of 55-atom icosahedral Cu₁₃Au_nPt_42-n, Au₁₃Cu_nPt_42-n and Pt₁₃Au_nCu_42-n (n = 1–41) trimetallic nanoalloys were investigated systematically. In order to present the composition effect on the melting behaviors in detail, three different icosahedral compositions corresponding to Cu₁₃Au₁₃Pt₂₉, Au₁₃Cu₂₉Pt₁₃ and Pt₁₃Au₂₉Cu₁₃ were selected from Cu₁₃Au_nPt_42-n, Au₁₃Cu_nPt_42-n and Pt₁₃Au_nCu_42-n nanoalloy systems by fixing two different type atom numbers at 13 and thus, melting process of Cu-rich, Au-rich and Pt-rich compositions was investigated. To analyze melting behavior of three compositions with different Cu, Au and Pt atom content, their best chemical ordering structures were used initially. Their melting behaviors were investigated thoroughly by using caloric curve, Lindemann index, heat capacity and RMSD methods.